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+An introduction to the videobuf layer
+Jonathan Corbet <corbet@lwn.net>
+Current as of 2.6.32
+
+The videobuf layer functions as a sort of glue layer between a V4L2 driver
+and user space. It handles the allocation and management of buffers for
+the storage of video frames. There is a set of functions which can be used
+to implement many of the standard POSIX I/O system calls, including read(),
+poll(), and, happily, mmap(). Another set of functions can be used to
+implement the bulk of the V4L2 ioctl() calls related to streaming I/O,
+including buffer allocation, queueing and dequeueing, and streaming
+control. Using videobuf imposes a few design decisions on the driver
+author, but the payback comes in the form of reduced code in the driver and
+a consistent implementation of the V4L2 user-space API.
+
+Buffer types
+
+Not all video devices use the same kind of buffers. In fact, there are (at
+least) three common variations:
+
+ - Buffers which are scattered in both the physical and (kernel) virtual
+ address spaces. All user-space buffers are like this, but it makes
+ great sense to allocate kernel-space buffers this way as well when it is
+ possible. Unfortunately, it is not always possible; working with this
+ kind of buffer normally requires hardware which can do scatter/gather
+ DMA operations.
+
+ - Buffers which are physically scattered, but which are virtually
+ contiguous; buffers allocated with vmalloc(), in other words. These
+ buffers are just as hard to use for DMA operations, but they can be
+ useful in situations where DMA is not available but virtually-contiguous
+ buffers are convenient.
+
+ - Buffers which are physically contiguous. Allocation of this kind of
+ buffer can be unreliable on fragmented systems, but simpler DMA
+ controllers cannot deal with anything else.
+
+Videobuf can work with all three types of buffers, but the driver author
+must pick one at the outset and design the driver around that decision.
+
+Data structures, callbacks, and initialization
+
+Depending on which type of buffers are being used, the driver should
+include one of the following files:
+
+ <media/videobuf-dma-sg.h> /* Physically scattered */
+ <media/videobuf-vmalloc.h> /* vmalloc() buffers */
+ <media/videobuf-dma-contig.h> /* Physically contiguous */
+
+The driver's data structure describing a V4L2 device should include a
+struct videobuf_queue instance for the management of the buffer queue,
+along with a list_head for the queue of available buffers. There will also
+need to be an interrupt-safe spinlock which is used to protect (at least)
+the queue.
+
+The next step is to write four simple callbacks to help videobuf deal with
+the management of buffers:
+
+ struct videobuf_queue_ops {
+ int (*buf_setup)(struct videobuf_queue *q,
+ unsigned int *count, unsigned int *size);
+ int (*buf_prepare)(struct videobuf_queue *q,
+ struct videobuf_buffer *vb,
+ enum v4l2_field field);
+ void (*buf_queue)(struct videobuf_queue *q,
+ struct videobuf_buffer *vb);
+ void (*buf_release)(struct videobuf_queue *q,
+ struct videobuf_buffer *vb);
+ };
+
+buf_setup() is called early in the I/O process, when streaming is being
+initiated; its purpose is to tell videobuf about the I/O stream. The count
+parameter will be a suggested number of buffers to use; the driver should
+check it for rationality and adjust it if need be. As a practical rule, a
+minimum of two buffers are needed for proper streaming, and there is
+usually a maximum (which cannot exceed 32) which makes sense for each
+device. The size parameter should be set to the expected (maximum) size
+for each frame of data.
+
+Each buffer (in the form of a struct videobuf_buffer pointer) will be
+passed to buf_prepare(), which should set the buffer's size, width, height,
+and field fields properly. If the buffer's state field is
+VIDEOBUF_NEEDS_INIT, the driver should pass it to:
+
+ int videobuf_iolock(struct videobuf_queue* q, struct videobuf_buffer *vb,
+ struct v4l2_framebuffer *fbuf);
+
+Among other things, this call will usually allocate memory for the buffer.
+Finally, the buf_setup() function should set the buffer's state to
+VIDEOBUF_PREPARED.
+
+When a buffer is queued for I/O, it is passed to buf_queue(), which should
+put it onto the driver's list of available buffers and set its state to
+VIDEOBUF_QUEUED. Note that this function is called with the queue spinlock
+held; if it tries to acquire it as well things will come to a screeching
+halt. Yes, this is the voice of experience. Note also that videobuf may
+wait on the first buffer in the queue; placing other buffers in front of it
+could again gum up the works. So use list_add_tail() to enqueue buffers.
+
+Finally, buf_release() is called when a buffer is no longer intended to be
+used. The driver should ensure that there is no I/O active on the buffer,
+then pass it to the appropriate free routine(s):
+
+ /* Scatter/gather drivers */
+ int videobuf_dma_unmap(struct videobuf_queue *q,
+ struct videobuf_dmabuf *dma);
+ int videobuf_dma_free(struct videobuf_dmabuf *dma);
+
+ /* vmalloc drivers */
+ void videobuf_vmalloc_free (struct videobuf_buffer *buf);
+
+ /* Contiguous drivers */
+ void videobuf_dma_contig_free(struct videobuf_queue *q,
+ struct videobuf_buffer *buf);
+
+One way to ensure that a buffer is no longer under I/O is to pass it to:
+
+ int videobuf_waiton(struct videobuf_buffer *vb, int non_blocking, int intr);
+
+Here, vb is the buffer, non_blocking indicates whether non-blocking I/O
+should be used (it should be zero in the buf_release() case), and intr
+controls whether an interruptible wait is used.
+
+File operations
+
+At this point, much of the work is done; much of the rest is slipping
+videobuf calls into the implementation of the other driver callbacks. The
+first step is in the open() function, which must initialize the
+videobuf queue. The function to use depends on the type of buffer used:
+
+ void videobuf_queue_sg_init(struct videobuf_queue *q,
+ struct videobuf_queue_ops *ops,
+ struct device *dev,
+ spinlock_t *irqlock,
+ enum v4l2_buf_type type,
+ enum v4l2_field field,
+ unsigned int msize,
+ void *priv);
+
+ void videobuf_queue_vmalloc_init(struct videobuf_queue *q,
+ struct videobuf_queue_ops *ops,
+ struct device *dev,
+ spinlock_t *irqlock,
+ enum v4l2_buf_type type,
+ enum v4l2_field field,
+ unsigned int msize,
+ void *priv);
+
+ void videobuf_queue_dma_contig_init(struct videobuf_queue *q,
+ struct videobuf_queue_ops *ops,
+ struct device *dev,
+ spinlock_t *irqlock,
+ enum v4l2_buf_type type,
+ enum v4l2_field field,
+ unsigned int msize,
+ void *priv);
+
+In each case, the parameters are the same: q is the queue structure for the
+device, ops is the set of callbacks as described above, dev is the device
+structure for this video device, irqlock is an interrupt-safe spinlock to
+protect access to the data structures, type is the buffer type used by the
+device (cameras will use V4L2_BUF_TYPE_VIDEO_CAPTURE, for example), field
+describes which field is being captured (often V4L2_FIELD_NONE for
+progressive devices), msize is the size of any containing structure used
+around struct videobuf_buffer, and priv is a private data pointer which
+shows up in the priv_data field of struct videobuf_queue. Note that these
+are void functions which, evidently, are immune to failure.
+
+V4L2 capture drivers can be written to support either of two APIs: the
+read() system call and the rather more complicated streaming mechanism. As
+a general rule, it is necessary to support both to ensure that all
+applications have a chance of working with the device. Videobuf makes it
+easy to do that with the same code. To implement read(), the driver need
+only make a call to one of:
+
+ ssize_t videobuf_read_one(struct videobuf_queue *q,
+ char __user *data, size_t count,
+ loff_t *ppos, int nonblocking);
+
+ ssize_t videobuf_read_stream(struct videobuf_queue *q,
+ char __user *data, size_t count,
+ loff_t *ppos, int vbihack, int nonblocking);
+
+Either one of these functions will read frame data into data, returning the
+amount actually read; the difference is that videobuf_read_one() will only
+read a single frame, while videobuf_read_stream() will read multiple frames
+if they are needed to satisfy the count requested by the application. A
+typical driver read() implementation will start the capture engine, call
+one of the above functions, then stop the engine before returning (though a
+smarter implementation might leave the engine running for a little while in
+anticipation of another read() call happening in the near future).
+
+The poll() function can usually be implemented with a direct call to:
+
+ unsigned int videobuf_poll_stream(struct file *file,
+ struct videobuf_queue *q,
+ poll_table *wait);
+
+Note that the actual wait queue eventually used will be the one associated
+with the first available buffer.
+
+When streaming I/O is done to kernel-space buffers, the driver must support
+the mmap() system call to enable user space to access the data. In many
+V4L2 drivers, the often-complex mmap() implementation simplifies to a
+single call to:
+
+ int videobuf_mmap_mapper(struct videobuf_queue *q,
+ struct vm_area_struct *vma);
+
+Everything else is handled by the videobuf code.
+
+The release() function requires two separate videobuf calls:
+
+ void videobuf_stop(struct videobuf_queue *q);
+ int videobuf_mmap_free(struct videobuf_queue *q);
+
+The call to videobuf_stop() terminates any I/O in progress - though it is
+still up to the driver to stop the capture engine. The call to
+videobuf_mmap_free() will ensure that all buffers have been unmapped; if
+so, they will all be passed to the buf_release() callback. If buffers
+remain mapped, videobuf_mmap_free() returns an error code instead. The
+purpose is clearly to cause the closing of the file descriptor to fail if
+buffers are still mapped, but every driver in the 2.6.32 kernel cheerfully
+ignores its return value.
+
+ioctl() operations
+
+The V4L2 API includes a very long list of driver callbacks to respond to
+the many ioctl() commands made available to user space. A number of these
+- those associated with streaming I/O - turn almost directly into videobuf
+calls. The relevant helper functions are:
+
+ int videobuf_reqbufs(struct videobuf_queue *q,
+ struct v4l2_requestbuffers *req);
+ int videobuf_querybuf(struct videobuf_queue *q, struct v4l2_buffer *b);
+ int videobuf_qbuf(struct videobuf_queue *q, struct v4l2_buffer *b);
+ int videobuf_dqbuf(struct videobuf_queue *q, struct v4l2_buffer *b,
+ int nonblocking);
+ int videobuf_streamon(struct videobuf_queue *q);
+ int videobuf_streamoff(struct videobuf_queue *q);
+ int videobuf_cgmbuf(struct videobuf_queue *q, struct video_mbuf *mbuf,
+ int count);
+
+So, for example, a VIDIOC_REQBUFS call turns into a call to the driver's
+vidioc_reqbufs() callback which, in turn, usually only needs to locate the
+proper struct videobuf_queue pointer and pass it to videobuf_reqbufs().
+These support functions can replace a great deal of buffer management
+boilerplate in a lot of V4L2 drivers.
+
+The vidioc_streamon() and vidioc_streamoff() functions will be a bit more
+complex, of course, since they will also need to deal with starting and
+stopping the capture engine. videobuf_cgmbuf(), called from the driver's
+vidiocgmbuf() function, only exists if the V4L1 compatibility module has
+been selected with CONFIG_VIDEO_V4L1_COMPAT, so its use must be surrounded
+with #ifdef directives.
+
+Buffer allocation
+
+Thus far, we have talked about buffers, but have not looked at how they are
+allocated. The scatter/gather case is the most complex on this front. For
+allocation, the driver can leave buffer allocation entirely up to the
+videobuf layer; in this case, buffers will be allocated with vmalloc_32()
+and will be very scattered indeed. If the application is using user-space
+buffers, no allocation is needed; the videobuf layer will take care of
+calling get_user_pages() and filling in the scatterlist array.
+
+If the driver needs to do its own memory allocation, it should be done in
+the vidioc_reqbufs() function, *after* calling videobuf_reqbufs(). The
+first step is a call to:
+
+ struct videobuf_dmabuf *videobuf_to_dma(struct videobuf_buffer *buf);
+
+The returned videobuf_dmabuf structure (defined in
+<media/videobuf-dma-sg.h>) includes a couple of relevant fields:
+
+ struct scatterlist *sglist;
+ int sglen;
+
+The driver must allocate an appropriately-sized scatterlist array and
+populate it with pointers to the pieces of the allocated buffer; sglen
+should be set to the length of the array.
+
+Drivers using the vmalloc() method need not (and cannot) concern themselves
+with buffer allocation at all; videobuf will handle those details. The
+same is true of contiguous-DMA drivers; videobuf will allocate the buffers
+(with dma_alloc_coherent()) when it sees fit. That means that these
+drivers may be trying to do high-order allocations at any time, an
+operation which is not always guaranteed to work. Some drivers play tricks
+by allocating DMA space at system boot time; videobuf does not currently
+play well with those drivers.
+
+Filling the buffers
+
+The final part of a videobuf implementation has no direct callback - its
+the portion of the code which actually puts frame data into the buffers,
+usually in response to interrupts from the device. For all types of
+drivers, this process works approximately as follows:
+
+ - Obtain the next available buffer and make sure that somebody is actually
+ waiting for it.
+
+ - Get a pointer to the memory and put video data there.
+
+ - Mark the buffer as done and wake up the process waiting for it.
+
+Step (1) above is done by looking at the driver-managed list_head structure
+- the one which is filled in the buf_queue() callback. Because starting
+the engine and enqueueing buffers are done in separate steps, it's possible
+for the engine to be running without any buffers available - in the
+vmalloc() case especially. So the driver should be prepared for the list
+to be empty. It is equally possible that nobody is yet interested in the
+buffer; the driver should not remove it from the list or fill it until a
+process is waiting on it. That test can be done by examining the buffer's
+done field (a wait_queue_head_t structure) with waitqueue_active().
+
+For scatter/gather drivers, the needed memory pointers will be found in the
+scatterlist structure described above. Drivers using the vmalloc() method
+can get a memory pointer with:
+
+ void *videobuf_to_vmalloc(struct videobuf_buffer *buf);
+
+For contiguous DMA drivers, the function to use is:
+
+ dma_addr_t videobuf_to_dma_contig(struct videobuf_buffer *buf);
+
+The contiguous DMA API goes out of its way to hide the kernel-space address
+of the DMA buffer from drivers.
+
+The final step is to set the size field of the relevant videobuf_buffer
+structure to the actual size of the captured image, set state to
+VIDEOBUF_DONE, then call wake_up() on the done queue. At this point, the
+buffer is owned by the videobuf layer and the driver should not touch it
+again.
+
+Developers who are interested in more information can go into the relevant
+header files; there are a few low-level functions declared there which have
+not been talked about here. Also worthwhile is the vivi driver
+(drivers/media/video/vivi.c), which is maintained as an example of how V4L2
+drivers should be written. Vivi only uses the vmalloc() API, but it's good
+enough to get started with. Note also that all of these calls are exported
+GPL-only, so they will not be available to non-GPL kernel modules.